1. Field of the Invention
The present invention relates to an image processing apparatus used in an image forming apparatus such as an MFP and to a copy machine control method.
2. Description of the Related Art
In recent years, techniques that exploit the visual sensory characteristics of humans to insert digital watermarks into the images of documents are being used in the realm of image formation. Inserting a digital watermark makes it possible to confirm a document's authenticity, display a warning indicating that copying the document is prohibited, add various types of auxiliary information, or carry out device control.
Conventionally, in copy machines provided with multiple functions, which are known as MFPs (multi-function peripherals), additional information is created using plural types of dots of differing sizes, and this additional information is synthesized with and embedded into the image of a document. For example, two types of dots with differing sizes are used, where a background is formed using the smaller dots, and embedded characters are formed using the larger dots. The background and embedded characters created using these two types of dots are synthesized with the image of a document as a ground design.
In order to prevent the unauthorized duplication of sensitive documents, ground designs, predetermined additional information, and the like, such as those described above, are added to the sensitive documents in advance.
For example, JP H06-20027A proposes an image processing apparatus capable of preventing the unauthorized distribution of sensitive documents, in which in the case where a determination means scans additional information of a document and has determined that the document is a sensitive document, a control means controls the copying operation of the document so that blank paper is outputted.
Meanwhile, JP 2006-295857A proposes an image forming apparatus in which, when a color duplicate of a sensitive paper document on which a ground design has been printed is made, a background portion of the ground design is caused to disappear, thereby causing a latent image portion of the ground design to appear with clarity. The image forming apparatus is thus capable of implementing duplication restriction effects provided by the ground design.
Furthermore, JP 2007-19734A proposes an image forming apparatus in which a latent image pattern composed of a character string, a pattern, or the like can be distinguished from a background pattern and appear in a clearly visible state, even if a background removal process, which takes the background pattern as a base pattern and deletes it, has been turned off by a user.
However, according to the methods disclosed in the documents described above, the latent image is reproduced in the copy, and thus characters such as “copying prohibited” or the like will appear in the ground design, even if the user is an authorized user. This runs contrary to the original intent of the authorized user.